Semiconductor memory device capable of switching between an asynchronous normal mode and a synchronous mode and method thereof

ABSTRACT

A semiconductor device comprises a memory cell array, a control section and a latency setting circuit. The control section configured to receive a clock signal and a control signal, and configured to output a plurality of data in synchronism with the clock signal after the control signal is asserted. The latency setting circuit configured to set the latency N, and the latency setting circuit including at least one switch which fixes the latency by use of an externally supplied signal.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

Japan Priority Application 04-063844, filed Mar. 19, 1992, including thespecification, drawings, claims and abstract, is incorporated herein byreference in its entirety. This application is a Divisional of U.S.application Ser. No. 10/642,624, filed Aug. 19, 2003 now U.S. Pat. No.6,842,397, incorporated herein by reference in its entirety, which is aContinuation of U.S. application Ser. No. 10/310,797, filed Dec. 6, 2002now U.S. Pat. No. 6,639,869, incorporated herein by reference in itsentirety, which is a Continuation of U.S. application Ser. No.09/983,383, filed Oct. 24, 2001 now U.S. Pat. No. 6,510,101,incorporated herein by reference in its entirety, which is aContinuation of U.S. application Ser. No. 09/435,627, filed Nov. 8, 1999now U.S. Pat. No. 6,310,821, incorporated herein by reference in itsentirety, which is a Continuation of U.S. application Ser. No.09/113,570, filed Jul. 10, 1998 now U.S. Pat. No. 5,986,968,incorporated herein by reference in its entirety, which is a Division ofU.S. application Ser. No. 08/457,165, filed Jun. 1, 1995 now U.S. Pat.No. 5,818,793, incorporated herein by reference in its entirety, whichis a File Wrapper Continuation of U.S. application Ser. No. 08/031,831,filed Mar. 16, 1993 now abandoned, incorporated herein by reference inits entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a clock-synchronous semiconductormemory device and access method thereof which operates synchronouslywith a basic clock signal, and, in particular, to a clock-synchronoussemiconductor memory device and access method thereof in which anaddress for accessing can be set synchronously with a basic clocksignal, and a clock-synchronous semiconductor memory device and accessmethod thereof in which an address for accessing can be set when ahigh-frequency basic clock signal is used.

2. Description of the Prior Art

The inventors of the present invention have previously proposed a basicmethod for controlling a memory operation for a semiconductor memorydevice synchronized with a basic clock signal (Japan Application No.3-255354).

At that time, several methods were illustrated for controlling a memoryaccess by means of an external control signal, but nothing was disclosedhow to set a external control signals synchronously with a basic clocksignal and with respect to setting specific timing for an address signalor the like for the external control signals.

Moreover, there is a problem that it is difficult to access data when ahigh-frequency basic clock signal is used in a conventional aclock-synchronous semiconductor memory device and access method thereof.

BRIEF SUMMARY OF THE INVENTION

According to one aspect of the present invention, there is provided amethod of switching modes of a memory system in which a normal mode anda synchronous mode are switchable, comprising supplying a clock signal,a first control signal and a second control signal to the memory system,setting the mode to be a normal mode in a case in which the secondcontrol signal is of a first level, and setting the mode to be asynchronous mode in a case in which the second control signal is of asecond level, when the first control signal transits, fetching in anaddress signal regardless of the clock signal when the mode is set to bethe normal mode, designating an address of a memory cell in a memorycell array after the address signal is asserted, inputting/outputtingdata regardless of the clock signal after the address of the memory cellin the memory cell array is designated in the normal mode, designatingthe address of the memory cell in the memory cell array in response toat least one of a rising edge and a falling edge of the clock signal,and obtaining a plurality of output data output from the memory cellarray in synchronism with the clock signal after the address signal isasserted, output of the data beginning a number of clock cycles (latencyN, N being a positive integer ≧2) of the clock signals, a different oneof the data being output at each of the clock cycles after the outputbegins until the plurality of data is output in the synchronous mode.

Moreover, according to another aspect of the present invention, there isprovided a method for switching operation of a memory system, comprisingsupplying a clock signal, a first control signal, and a second controlsignal to a memory system, designating an address of a memory cell in amemory cell array regardless of the clock signal, in a case in which thesecond control signal is of first level when the first control signal istransited, inputting/outputting data regardless of the clock signalbased on the designated address, after an address signal is asserted,designating an address of the memory cell in the memory cell array insynchronism with the clock signal, in a case in which the second controlsignal is of second level when the first control signal is transited,and obtaining a plurality of output data output from the memory cellarray in synchronism with the clock signal after the address signal isasserted, output of the data beginning a number of clock cycles (latencyN, N being a positive integer ≧2) of the clock signals, a different oneof the data being output at each of the clock cycles after the outputbegins until the plurality of data is output.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an external signal waveform diagram showing aclock-synchronous operation of a clock-synchronous semiconductor memorydevice relating to an access method according to the first embodiment ofthe present invention.

FIG. 2 is an external signal waveform diagram for anaddress-incorporated nonsynchronous-type of clock-synchronous method fora clock-synchronous semiconductor memory device relating to an accessmethod which is another embodiment of the present invention.

FIG. 3 is a chart showing an example of external signal waveforms forswitching between a normal access mode and the clock synchronous mode ofthe present invention.

FIG. 4 is a chart showing an example of external signal waveforms forswitching between a normal mode and a conventional clock synchronousmode according to the present invention.

FIG. 5 is a chart comparing external waveforms in the case of modifyingthe number of clock cycles used in the internal operation, within thesame memory.

FIG. 6 is a configuration diagram for a clock-synchronous semiconductormemory device of the present invention which is capable of executing theaccess methods shown in FIGS. 1 to 5.

FIG. 7 is a diagram of a clock-synchronous delay circuit for an internalcircuit drive signal.

FIG. 8 is a circuit diagram for a delayed signal selection switchingcircuit.

FIG. 9 is a circuit diagram for a blown fuse signal generating circuit.

FIG. 10 is a circuit diagram for a circuit for generating a drive signalfor a delayed signal selection switching circuit.

FIG. 11 is a waveform diagram showing the relationship between eachdelayed signal in FIG. 8 and the basic clock cycle.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The features of this invention will become apparent in the course of thefollowing description of exemplary embodiments which are given forillustration of the invention and are not intended to be limitingthereof.

Preferred embodiments of the present invention will now be explainedwith reference to the drawings.

This example will be explained using a timing chart illustrated in FIG.1.

In the timing chart of FIG. 1, all of the signals are set with respectto the transition of the rising edge of the basic clock signal CLK. Forexample, at a first clock cycle CLK1, an external signal /RE, which isprovided from external device, is switched to the “L” level, and aso-called row address which designates a row in a memory cell array isintroduced. Accordingly, the setting of the conditions of this address,as shown in the drawing, is asserted based on the rising edge of thebasic clock signal CLK.

In the same manner, a column address designating a column in the memorycell array is based on the transition of the falling edge of a clockcycle (CLK4) of the basic clock signal CLK when the external controlsignal /CE is at “L” level, specified as shown in the drawing.

In a data output operation, for example, a series of access operationsis carried out in three clock cycles, and at the fourth cycle (CLK8)data (C1) is output to the outside from the chip in which asemiconductor memory device is formed.

In changing the column address during the course of the serial output, acycle in which the Column Enable signal /CE as a control signal is at“L” level is made, and the column address is set in the same manner withrespect to the timing of the transition of the rising edge (CLK15) ofthis basic clock signal. After the four cycles later from the setting(CLK19), data is output serially in a predetermined order (C2, C2+1,C2+2, . . . ), in which the data at a new column address is output atfirst.

The case will be considered where the basic clock signal period isshort, for example, about 10 ns. In this case, it is difficult to set anaddress from a particular cycle synchronous with this basic clock signalCLK, while reliably setting the set-up and holding time for maintainingthe conditions of the address signal, based on the transition of therising edge of one cycle of the basic clock signal CLK. Specifically, itis difficult, counting from the next cycle, to select the specifiedcycle and to set the address within that cycle.

In addition, from an aspect of a circuit operation, it is difficult tospecify a cycle and reliably input an address signal or the like in thiscycle, then operate this circuit stably with good reliability when theperiod of the basic clock signal CLK is short. Strict timing control isnecessary for both the system aspect side and the chip aspect side, anda complicated circuit design is required to provide this.

Moreover, in the case of a system where the period of the basic clocksignal CLK is long, when the memory chip normally performs an internaloperation at a specified cycle following the setting cycle for thecolumn address, a large amount of access time is necessary for accessingthe head when accessing from a newly-set column address.

In this manner, when an operating method utilizing a basic clock signalCLK is uniform, or more specifically, invariable, the system cycle timeis unchangeable to a certain degree.

There is therefore the problem that this operating method is difficultto apply in practice to a system in a range which can efficientlyutilize the cycle.

To solve the problem, the second preferred embodiment of the presentinvention will now be explained.

There is a case that it is difficult to set an address in one cyclesynchronous with a basic clock signal CLK when the cycle time is short.

One method of avoiding this is provided in the embodiment of the presentinvention shown in FIG. 2.

In this drawing, first, when a Row Enable control signal /RE is at “L”level (immediately prior to a signal CLK1), the operation forintroducing the address is activated. However, at this time, the addressoperation inside a semiconductor memory device synchronized with thebasic clock signal CLK has not yet started. The input of this address isthe same as setting by means of a conventional DRAM, and in the settingof the address there are no occasional difficult areas. Specifically,the address can be introduced without restriction in the timing of thebasic clock. In this manner, the address is set by means of the RowEnable control signal /RE and a Column Enable control signal /CE (whichare referred to as first control signals in claim 2), and actual dataaccess for an address introduced into memory synchronized with the basicclock signal CLK is commenced by means of a control signal /SYNC as anexternal second control signal. If the control signal /SYNC is at “L”level (CLK4) when the transition of the onset of the clock signal, itenters the synchronous mode from that cycle, and an internal accessoperation proceeds synchronous with the basic clock signal.

As a result, the output of data C1 to the outside begins at thecommencement of a synchronized operation (CLK4) in this embodiment atthe fourth cycle (CLK8). To change the column address during serialaccess, with the control signal /SYNC at “H” level (CLK12), a new columnaddress C2 is introduced asynchronously with the basic clock signal, andan address is set and introduced at the falling edge of the controlsignal /CE (CLK12). Next, the control signal /SYNC is once againswitched to “L” level (CLK15) and synchronized access commences at thisnew address.

In FIG. 2, access to a new column address starts from CLK15, and aswitch from address C1 to address C2 is made from CLK19 after fourcycles.

On further expanding this concept, it is possible to provide asemiconductor memory device in which an operation mode can be set foreach cycle which sets a row address. The operation mode in this casemeans data output timing and is defined as either a basic clock signalsynchronized access mode (hereinafter synchronous mode) wherein the dataoutput is accessed in synchronism with a state of the basic clock signalCLK after addresses to be accessed are set or a mode in which dataoutput is started after addresses to be accessed are set like aconventional DRAM (hereinafter we call the mode “a normal mode”).

FIG. 3 and FIG. 4 are charts showing a method of switching between thesetwo modes.

FIG. 3 illustrates a method for switching between a conventional normalmode and the synchronous mode of the present invention explained in thesecond embodiment shown in FIG. 2. The control signal /SYNC is used as acontrol signal for this switching. If this control signal /SYNC is at“L” level when the control signal /RE falls (CLK1), the normal mode isin effect; if at “H” level, it is the same as the normal mode foraddress input operation, but the synchronous mode is effected only whenthe control signal /SYNC is fallen (CLK31). This is an example for thesynchronous mode in the present invention.

FIG. 4 shows the case of switching between a conventional normal modeand the synchronous mode illustrated in the first embodiment shown inFIG. 1. In this case, for example, the control signal /SYNC is used, andthe condition of the control signal /SYNC when the control signal /REfalls determines whether the normal mode or the synchronous mode isentered.

In FIG. 4, the normal mode occurs when the control signal /SYNC is at“H” level; and the synchronous mode occurs when the control signal /SYNCis at “L” level (CLK22). When the control signal /RE is at the “L”level, synchronous operation commences from the first clock signal(CLK23). In the switch to this mode, it is obvious that, even when thecontrol signal /SYNC is not used, the mode setting cycle may be setseparately.

In any of the above cases, because it is possible to provide for aconventional normal mode and a synchronous mode of the present inventionby using a time sharing method in the same semiconductor memory device,this method is effective in the case where random access and high speedserial access are required in the same semiconductor memory device.

Next, a case will be explained in which the basic clock signal CLK ofthe system is not necessarily produced at maximum speed. If at a cycletime of 10 ns there is efficient synchronous memory control, while at acycle time of 20 ns the operation within the memory remains unchanged,the initial access after setting the column address requires twice thetime.

Also the time to spare for the operation within the memory becomes largeso that there is considerable time during which the operation is idle.

In order to avoid this and achieve an efficient memory operation, it isdesirable that the cycle of the internal operation be capable ofmodification according to the cycle length of the basic clock signal CLKused.

FIG. 5 is a timing chart showing an example of an access timing methodwith the above-mentioned type of function. This chart shows two cases,each with a different number of cycles required for the internaloperation. Also, an example of a conventional type of synchronous modeis illustrated in order to make the explanation more easilyunderstandable.

Example 2 in FIG. 5 illustrates the case where the number of cycles ofthe synchronous operation corresponds to the first embodiment shown inFIG. 1.

Example 1 in FIG. 5 corresponds to a case in which the number of cyclesof the synchronous operation Is decreased. In this case, an operationfollowing the internal operation of example 1 cannot be carried out at ashort period for a basic clock cycle the same as example 2, but thechart shows two cases with respect to the same clock cycle in order toobserve the difference in the number of control cycles. In example 1,the internal operation is performed in half the number of cycles ofexample 2. Accordingly, in practice, example 2 relates to control of asystem in which the period of the basic clock signal is a 10 ns cycle,while example 1 shows a system control method for a 20 ns cycle.

An optimum operation is performed with both these systems. Anotherembodiment of the present invention will now be explained.

A configuration of a clock-synchronous semiconductor memory device whichcan execute the access methods shown in FIGS. 1 to 5 described abovewill be explained referring to FIG. 6.

FIG. 6 shows the configuration of the clock-synchronous semiconductormemory device 10 which can execute effectively the access methods of thepresent invention.

As one of basic operation of the semiconductor memory device 10, amemory access operation of the semiconductor memory device 10 is carriedout based on an external basic clock signal CLK and at least one or moreexternal control signals which are provided continuously to thesemiconductor memory device.

In FIG. 6, a memory device 10 comprises a counting section 5 and acontrol section 14, which are main control elements of the semiconductormemory device, in addition to a memory cell group 11, a selectionsection 12, a specification section 13.

A dynamic memory cell, a static memory cell, or the non-volatile memorycell of the memory cell group 11 is arranged in the form of a matrix.

The data which is written in and read out is stored in this memory cell.

The data access is carried out between the memory cell group 11 andexternal devices (not shown) through a data I/O section 4.

The specification section 13 sets consecutive addresses in the memorycell group 11 according to a series of externally-provided addresssignals under the control of the control section 14, and designates, inorder, the memory cells which are to be accessed. Under the control ofthe control signals /SYNC, /RE, and /CE input into the control section14, the specification section 13, for example, fetches a row addresssignal, then fetches a series of column address signals for a string ofmemory cells in the memory cell group 11 connected to a word linedesignated by the row address signal. The specification section 13designates a series of memory cells consecutively by means of the columnaddress signal.

The data I/O section 4 performs a read or write operation on the memorycell group 11 designated by the specification section 13 based on aread/write signal obtained externally.

The read-out data is output to an external destination through the dataI/O section 4. The data to be stored is provided to the designatedmemory cell from an external source through the data I/O section 4 bythe specification section 13.

The counting section 5 is a counter for counting the number of cycles ofthe basic clock signal CLK continuously input at an almost fixedfrequency from an external source.

The counter 5 is capable of counting a fixed number of clock cycles ofthe basic signal CLK and discriminating some clock cycles from othercycles. A circuit essentially having the function can be considered asthe counter 5. Therefore a circuit having the function described abovecan be used instead of the counter 5 when there is the circuit in thesemiconductor memory device 10.

The external basic clock signal CLK used in this embodiment is a clocksignal with a cycle time of, for example, less than the 30 ns accesstime of the memory device. The counting section 5 provides the controlsection 14 with the count of the number of cycles of the clock signalCLK.

The control section 14 sends a selection signal to the selection section12 based on the level of the control signal /SYNC provided fromexternal.

Based on the selection signal, the selection section 12 selects theaccess timing of the memory cell group 11, then sends address activationsignal φA to φD to the memory cell group 11.

The selection section 12 selects either the normal operation mode or thesynchronous operation mode which have been already explained and shownin FIGS. 3 and 4 under the control of the control section 14.

When the selection section 12 is not included in the semiconductormemory device 10, the semiconductor memory device performs only theaccess methods shown in FIGS. 1 to 4.

Configurations of the counting section 5 and the control section 14 willbe described below referring to FIGS. 7 to 10.

Generally, the internal operation is basically controlled with a minimumunit of operation time corresponding to a basic clock signal CLK.Accordingly, the number of basic clock cycles it takes to carry out aseries of operations can be selectively determined by controlling thetransmission to the section of the circuit in which this operation iscarried out, using a number of cycles for a signal to start a certainoperation.

FIGS. 7 to 10 show examples of configurations of circuits by which thisselection can be set by using an external laser to blow a fuse inside achip.

FIG. 7 shows an example in which a trigger signal RINT for a certaincircuit is delayed only for a time corresponding to a number of parts ofthe cycles of the basic clock signal CLK. This circuit is a so-calledshift register circuit, and transmission is carried out successively inthe state where the signal RINT is at the “H” level, according to thechange in a signal INTCLK inside the chip, synchronized with the basicclock signal CLK. INTCLK and /INTCLK are of opposite phases. In FIG. 7,when /INTCLK is at the “H” level, a signal in a latch circuit of theprevious step is transmitted, and when INTCLK is at the “H” level, asignal in a latch circuit of the following step is transmitted.Accordingly, a delay circuit in FIG. 7 produces a signal delay at onepart of the basic clock cycle, and the signal RINT is delayed by onecycle and output as a signal CINT1.

In addition, by passing through the same type of circuit, CINT2 isproduced which is a delay of one cycle from CINT1, and CINT3 is producedwhich is a delay of one cycle from CINT2. In a clocked invertor such asshown in FIGS. 7 and 8, the circuit acts as an invertor at the “H” levelsignal expressed at the output part and at the “L” level signalexpressed at the output part, and the output becomes a high impedanceand is isolated from a node portion proceeding the output. The relationof the basic clock cycle to the signal is shown in FIG. 11. In thischart, a plurality of signals CINT1, CINT2, and CINT3 is shown, each ofwhich onsets at the respective cycles CLK2, CLK3, and CLK4, which aresuccessively one cycle delayed respectively from the signal RINT whichis risen at the signal CLK1. Depending on which of these signals isused, it is possible to specify at which cycle following a prescribedcycle in the basic clock signal certain, operations, for example, I/Ooperations, will be performed. When observed from a basic configurationportion of a synchronous-type memory, these delayed circuits can beconsidered as forming counters for the basic clock cycle.

FIG. 8 is a diagram showing a part which selects any delayed signal andsupplies this signal to a driven circuit as the signal CINT used inactual control. From the action of the clocked invertor, the signaloutput as the output signal CINT when VL is at the “H” level is CINT1;when VM is at the “H” level—CINT2; and when VH is at the “H”level—CINT3. The circuit used as the switch, if switched in accordancewith the period of the basic clock signal CLK of the system using thememory, can cause the optimum operation to be performed in the system.

Several methods for creating the signal for switching can be considered.Blowing a fuse; modifying a mask pattern for a process for including awiring layer in the memory IC; a method by which an internal node isgiven either a floating or a fixed potential using bonding from a powersource line pin, which has the same effect as blowing a fuse; a methodfor distinguishing whether a pin used as a non-connected pin isconnected to the power source or is floating, or the like; a programmingmethod for distinguishing the condition of another external signal atthe timing when the control signal /RE falls or the like; are exampleswhich can be given.

The following explanation covers a specific case using the blowing of afuse. FIG. 9 is a diagram showing a circuit for creating a combinationof four signal conditions by blowing two fuses. In the case whereneither a fuse 1 or a fuse 2 is blown, a signal F1 and a signal F2 areset at the “L” level until the onset of the signal RINT, then, at theonset of the signal RINT both the signals F1 and F2 rise to the “H”level.

On the other hand, when a fuse is blown, because a transistor T1 or atransistor T2 does not become a pass connected to ground or earth, thesignal F1 or the signal F2 is maintained at the latch level and is heldat the “L” level even on the onset of the signal RINT.

According to the method of blowing the fuses 1, 2, there are four waysin which conditions of the signals F1 and F2 can be combined.

Three of these four ways for creating a signal for input to theswitching circuit of FIG. 8, are illustrated by the circuits shown inFIG. 10. The circuits shown in FIG. 10 are logical circuits for creatingthe signals VH, VM, and VL from the signals F1 and F2 produced by thecombination of the blow of the fuses when the signal RINT is at the “H”level. If neither of the fuses 1 and 2 in the circuits explained aboveis blown, VH is switched to the “H” level and the onset of the signalCINT occurs at the fourth cycle from the onset of the signal RINT. Ifthe fuse 1 only is blown, VM is switched to the “H” level and the onsetof the signal CINT occurs at the third cycle from the onset of thesignal RINT. When both fuses are blown, VL is switched to the “H” leveland the onset of the signal CINT occurs at the second cycle from theonset of the signal RINT.

In the case where the fuse 2 only is blown, none of the signals onsets,therefore, the signal CINT does not onset.

In all methods such as modifying a mask pattern for a process forintroducing another wiring layer in the memory IC; a method usingbonding from a power source line pin to a pad for an internal node inplace of a fuse, and a method for distinguishing whether a pin used as anon-connecting pin is connected to the power source or is floating, orthe like, the structure and the method for grounding the correspondingnode of the transistors T1, T2 in place of the fuses 1, 2 can be easilyinferred by one skilled in the art. These particulars are self-evident,therefore further explanation will be omitted here.

On the other hand, in a programming, method for distinguishing thecondition of several external signals at the timing when the controlsignal /RE falls, or the like, signals corresponding to the signals F1,F2 are created directly by the internal logic. If the correspondingrelationship with the external signal condition is set, it is possibleto easily fabricate a logic circuit so that a signal corresponding to F1and F2, or VH, VM and VL is generated during that condition.

As explained in the foregoing, with the clock-synchronous semiconductormemory device of the present invention, for example, in the case wherethe address is set in synchronism with the basic clock signal CLK afterthe control signals /RE and /CE are input, memory access operation canbe carried out accurately.

Further, for example, in the case where the period of the basic clocksignal CLK for the system is short, it is possible to set an addressvalue using a method unrelated to the length of the period of the basicclock cycle. Accordingly, the design of the system timing and theprerequisites relating to the internal memory operations becomes easier,even in the case where the period of the clock cycle is short.

Further, with respect to access of data, the present invention takesadvantage of the special feature of the clock synchronous method ofaccess.

In addition, when random access such as the page mode of a conventionalDRAM is necessary, and also in the case where the system is based on acircuit structure in which high speed serial access is synchronous withthe clock cycle, it is possible to switch between DRAM mode andsynchronous mode on the same chip through time-sharing. Therefore, ifother methods are used in the present invention, the system can beefficiently constructed.

In particular, it can be applied in practice to an image memory.Furthermore, in order to cope with optimum operation of memory insystems with various periods shown in other embodiments, it is possibleto modify the number of cycles used for the data access operation of thememory, therefore, it is possible to design a single memory forapplication to many systems. For this reason, a memory can be selectedwhich can demonstrate system performance of maximum scope.

Thus, it is possible to certainly set an address to be accessed, inspite of the length of a period of a basic clock signal, and to outputdata accurately by the clock-synchronous semiconductor memory device andaccess methods thereof according to the present invention.

1. A method of switching modes of a memory system in which a normal modeand a synchronous mode are switchable, comprising: supplying a clocksignal, a first control signal and a second control signal to the memorysystem; setting the mode to be the normal mode in a case in which thesecond control signal is of a first level, and setting the mode to bethe synchronous mode in a case in which the second control signal is ofa second level, when the first control signal transits; fetching in anaddress signal regardless of the clock signal when the mode is set to bethe normal mode; designating an address of a memory cell in a memorycell array after the address signal is asserted; inputting/outputtingdata regardless of the clock signal after the address of the memory cellin the memory cell array is designated in the normal mode; designatingthe address of the memory cell in the memory cell array in response toat least one of a rising edge and a falling edge of the clock signal;and obtaining a plurality of output data output from the memory cellarray in synchronism with the clock signal after the address signal isasserted, output of the data beginning after a number of clock cycles(latency N, N being a positive integer ≧2) of the clock signals, adifferent one of the data being output at each of the clock cycles afterthe output begins until the plurality of data is output in thesynchronous mode.
 2. The method according to claim 1, wherein a level ofthe second control signal except a case where the first control signaltransits affects none of the operation in the mode.
 3. The methodaccording to claim 1, wherein, in the inputting/outputting of dataregardless of the clock signal, initiation of inputting/outputting ofdata is executed after a predetermined time has passed since the addressis designated.
 4. The data according to claim 1, further comprisingsupplying a read/write signal to the memory system so that dataoutput/input is carried out in accordance with the read/write signal. 5.The method according to claim 1, wherein the asserted address signal toobtain the plurality of output data is a column address signal.
 6. Amethod for switching operation of a memory system, comprising: supplyinga clock signal, a first control signal, and a second control signal tothe memory system; designating an address of a memory cell in a memorycell array regardless of the clock signal, in a case in which the secondcontrol signal is of a first level when the first control signal istransited; inputting/outputting data regardless of the clock signalbased on the designated address, after an address signal is asserted;designating an address of the memory cell in the memory cell array insynchronism with the clock signal, in a case in which the second controlsignal is of a second level when the first control signal is transited;and obtaining a plurality of output data output from the memory cellarray in synchronism with the clock signal after the address signal isasserted, output of the data beginning after a number of clock cycles(latency N, N being a positive integer ≧2) of the clock signals, adifferent one of the data being output at each of the clock cycles afterthe output begins until the plurality of data is output.
 7. The methodaccording to claim 6, wherein the second control signal is used only forswitching of operations.
 8. The method according to claim 6, whereininitiation of said inputting/outputting data regardless of the clocksignal is executed after a predetermined time has passed since theaddress signal is asserted.
 9. The method according to claim 6, furthercomprising supplying a read/write signal to the memory system so thatdata output/input is carried out in accordance with the read/writesignal.
 10. The method according to claim 6, wherein the assertedaddress signal to obtain the plurality of output data is a columnaddress signal.